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Re: [Xen-users] issue with iptables antispoofing rules in xen4.8 generetab by vif-bridge and vif-common.sh

Hi Sebastian,

On 03/24/2018 04:03 PM, spi@xxxxxxxxx wrote:
> Hi all
> I filed this issue with the Debian user-list as well but as I think it is not 
> Debian related I file it here as well.

What you can do best (for the Debian part of it) is file a bug against
src:xen in Debian to start a discussion.

The behaviour will likely not change any more for Xen 4.8 in Stretch,
but this is one of the things that should be improved for Xen 4.1X in
Buster. (I'm one of the people working on that.)

> I have issues with the on domU startup automatically generated
> antispoofing rules by
> /etc/xen/scripts/vif-bridge and
> /etc/xen/scripts/vif-common.sh

I myself use openvswitch instead of bridges, because it's so much more
convenient with vlans and in that situation all of the rules do not make
sense anyway, since since openvswitch traffic to domUs does not hit

> Both are part of the Debian xen-utils-common package
> (4.8.3+comet2+shim4.10.0+comet3-1+deb9u5 installed on Debian 9.4).
> A domU test01 has two virtual interfaces - vif-test01-INT and
> vif-test01-TEST, both are connected to separate bridges named brINT and
> brTEST. The brINT is just an internal bridge without connectivity to an
> outside network to just connect all domUs and the dom0. The IP
> addressfor the vif-test01-INT interface is
> The automatically generated rules per domU are:
> 1    ACCEPT     all  --  anywhere             anywhere            
> PHYSDEV match --physdev-out vif-test01-INT --physdev-is-bridged
> [...]

Because of the above, I have actually never studied this behaviour in
detail, I've just always patched it out, postponing looking for a proper
fix. (My network at work only contains domUs which are managed by
ourselves, noone else has shell/root access on them).

> From what I see is that the rules 1/3 and 5/7 are doubled.
> The next issue is that antispoofing rules just don't work. If I change
> the ip adress of the vif-test01-INT interface to something like
> IP packets between test01 and other domUs are still
> forwarded.
> If I manually change the iptables rules to something like (in this
> example just for the brINT connected interface):
> -A FORWARD -m physdev --physdev-is-bridged --physdev-in vif-test01-INT
> -p all ! -s -j DROP
> -A FORWARD -m physdev --physdev-is-bridged --physdev-out vif-test01-INT
> -p all ! -d -j DROP
> -A FORWARD -m physdev --physdev-is-bridged --physdev-in vif-test01-INT
> -p all -j ACCEPT
> -A FORWARD -m physdev --physdev-is-bridged --physdev-out vif-test01-INT
> -p all -j ACCEPT
> ...
> -A FORWARD -j REJECT --reject-with icmp-port-unreachable
> then antispoofing works and IP packets with IP addresses different then
> get dropped.
> Can somebody confirm this is an issue? Or do I just not understand how
> the antispoofing rules work on a virtual bridge?

The general feeling I have is that this area indeed needs some work to
clean it up and make it better. TBH, I have not looked back yet in xen
version control to see what the history of these scripts and the
iptables changes is.

Taking one step back, what are the things we want to do with whatever
kind of rules in the dom0?
* filtering irrelevant L2 unicast flooding
* whitelisting mac addresses
* whitelisting ip addresses
* ?

For bridges, some of these things can be done with iptables (if you also
set the right sysctl options to actually make bridged traffic hit
iptables...), but for filtering specific mac address / ip address etc
you need to know about them in the dom0. For openvswitch, it can
probably also be done in a different way. Then, what about properly
cleaning up things on domU shutdown, what about live migration, other

So, there's not really a good out of the box solution for this now
afaics. We should find out what's there now, and why it was done like
that in the past and then see how it can be improved.

Your experience, opinion, feedback etc. matters. Thanks for starting
this thread. Best thing is to do the improvements "upstream" (here)
instead of in Debian.

> Is there a way to diable generation of antispoofing rules automatically
> on domU startup? I could configure a different vif.default.script in
> xl.conf and write a wrapper script, but it might be easier to just
> disable it and load iptables rules manually.

Remove/comment-out the handle_iptable line.


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